Various processes known in the prior art for obtaining lead from the sludge of exhausted batteries begin with the step of desulfurizing the sludge by treating it with solutions of alkali-metal or ammonium carbonate. In this treatment, practically all sulfur contained in the battery sludge passes into solution in the form of soluble alkali-metal sulfate or ammonium sulfate, and a desulfurized paste, also called the "active mass", containing a mixture of insoluble lead compounds is obtained.
Currently, pyrometallurgical reduction is the most widely used method for extracting lead from desulfurized paste. However, pyrometallurgical processes have the disadvantage that special precautions must be taken in handling lead-containing materials in a furnace to avoid spreading lead fumes and dust. Extensive filtration facilities and monitoring equipment are required to detect harmful lead contaminants and to avoid spreading them in the workplace and environment. Moreover, pyrometallurgical processing produces a slag that is, due to environmental concerns, expensive to dispose of.
Hydrometallurgical processes, such as electrowinning, constitute a valuable alternative to pyrometallurgical processes by avoiding many of the problems of pollution described above. However, with electrowinning of lead compounds there arises the problem that lead dioxide (PbO.sub.2), a major component of the desulfurized paste of battery sludge, is insoluble in the normal acids suitable for electrowinning. Electrowinning without converting lead dioxide into a soluble compound produces a low yield and leaves a toxic, lead-containing residue.
A variety of techniques for the solubilization of lead compounds of an active mass are taught in the prior art as shown by the following patents:
C. E. Tucker in U.S. Pat. No. 1,148,062 discloses heating of the battery sludge in order to transform PbO.sub.2 into soluble PbO and Pb.sub.2 O.
W. C. Smith in U.S. Pat. No. 1,752,356, in order to solubilize PbO.sub.2 before the attack with caustic alkali, treats the whole active mass by a heating step under a reducing atmosphere (PbO is formed).
J. H. Calbeck in U.S. Pat. No. 1,911,604 provides for the active mass of the battery to be leached by a solution of sodium acetate. Pb oxide and sulfate are dissolved, while PbO.sub.2 is normally insoluble in that electrolyte. But, in the presence of metal Pb and in the said electrolyte, a local couple is established, so that PbO.sub.2 and an equivalent amount of metal Pb should be dissolved.
A. F. Gaumann in U.S. Pat. No. 4,107,007 leaches the active mass with a concentrated solution of an alkali-metal hydroxide, to which molasses, or raw sugar, or similar products, has been previously added. In such way, Pb oxide and Pb sulfate are dissolved, and are sent to the electrolysis. The behavior of PbO.sub.2 is not detailed.
M. E. Elmore in U.S. Pat. No. 4,118,219, in order to reduce PbO.sub.2, mentions the use of some reducing agents, such as formaldehyde, H.sub.2 O.sub.2, metal Pb, or calcination.
R. D. Prengaman in U.S. Pat. No. 4,229,271 proposes two routes for eliminating PbO.sub.2 from the active mass, and rendering it wholly soluble in the usual acids for the electrowinning process:
(a) a drying at 100.degree. C. of the active mass, followed by a roasting under a reducing atmosphere at temperatures comprised within the range of from 290.degree. to 325.degree. C.; PA1 (b) a treatment of the aqueous suspension of the active mass with sulfur dioxide, or with alkali-metal or ammonium sulfite or bisulfite. PA1 (a) by means of the addition of Pb powder during the leaching with fluosilicic acid of past already desulfurized by reaction with ammonium carbonate; PA1 (b) by means of the addition of ammonium bisulfite during the treatment of desulfurization with ammonium carbonate. PA1 (a) desulfurizing battery sludge to form a desulfurized paste; PA1 (b) leaching the desulfurized paste with an aqueous solution of an acid selected from the acids suitable for electrowinning to form a liquid/solid mixture; PA1 (c) separating the liquid/solid mixture by press-filtering and washing to form a filtrate containing Pb.sup.++ ions and an insoluble residue consisting essentially of lead dioxide organic substances and moisture; PA1 (d) treating the filtrate by electrowinning to produce lead in pure metal form thereby creating an exhausted electrolyte; PA1 (e) treating the insoluble residue formed in step (c) with concentrated sulfuric acid sufficient to cause the simultaneous occurring of the following reactions: EQU C.sub.n (H.sub.2 O).sub.m +H.sub.2 SO.sub.4 .fwdarw.nC+H.sub.2 SO.sub.4.mH.sub.2 O (1) EQU C+2PbO.sub.2 +2H.sub.2 SO.sub.4 .fwdarw.2PbSO.sub.4 +CO.sub.2 +2H.sub.2 O(2) PA1 (f) feeding the so-treated insoluble residue to the desulfurizing of step (a).
U. Ducati in U.S. Pat. No. 4,460,442 makes the active mass of the battery react at 100.degree.-120.degree. C. in the presence of a strongly alkaline solution, in order to obtain a precipitate of minium, which should display the property of getting completely dissolved in the hot concentrated solutions of fluoboric and fluosilicic acid.
A. Y. Lee and E. R. Cole of Bureau of Mines in R.I. 8857 suggest two ways for reducing PbO.sub.2 contained in the active mass:
The suggested methods of high-temperature reduction of PbO.sub.2 under a reducing atmosphere show the disadvantage that they add two steps to the processing cycle: the drying, and the reducing roasting. These processing steps require a strict control of the operating conditions, and furthermore must be carried out inside a unit (the furnace, or the roaster) provided with an adequate dust removal facility. Furthermore, even with low temperatures, the handling of a dry material may cause environmental pollution.
The method of reduction during acidic attack, by means of the addition of lead powder, implicitly requires the transfer of a portion of the produced lead to produce the powder from it; it is therefore expensive, not only due to the recycling of lead, but also due to a low reaction speed at temperatures close to ambient temperature.
The method of reduction with sulfur dioxide, sulfite or bisulfite before the carbonation step involves great expense due to the excess amount of reactant that must be added and increases the amount of carbonate required for the desulfurization by approximately 25%. The reaction is slow and the yield of PbO.sub.2 reduction is generally not total.
In general, all these systems known from the prior art suffer from the serious problem that they do not secure a total dissolving of the lead contained in the active mass of the batteries.
The failure using methods known in the prior art to achieve a substantially total recovery of the lead contained in battery sludge could be caused in part by the presence of the organic substances in the desulfurized paste. The organic substances include substances introduced during the manufacture of the batteries and substances such as fragments of separators, wood, fibers, and paper that get concentrated in the active mass during the crushing and processing of exhausted batteries. Inasmuch as most of these organic substances have a structure of cellulose type, and are very porous, they retain lead compounds, prevent them from being completely dissolved, eventually lowering the yield of the recovery process.
Accordingly, there exists a need for a hydrometallurgical process for recovering substantially all the lead from battery sludge that avoids the problems of pollution and expense of pollution control associated with pyrometallurgical processes and avoids the problems of inefficiency associated with known hydrometallurgical processes.